1. Field of the Invention
The present invention relates to a potential measurement apparatus of a non-contact type measuring potential of a measurement object in a non-contact manner and an image forming apparatus including the potential measurement apparatus.
2. Description of the Related Art
Conventionally, in an image forming apparatus including a photosensitive drum to form an image with an electrophotographic system, it is necessary to charge the photosensitive drum to appropriate (typically uniform) potential under any environment in order to obtain stable image quality. Therefore, by measuring the charged potential of the photosensitive drum with a potential measurement apparatus and utilizing the results thereof, feedback control is carried out so as to keep the potential of the photosensitive drum uniform.
As a potential measurement apparatus used for an image forming apparatus, the following has been proposed (Japanese Patent Application Laid-Open No. 2004-301555). FIG. 9A illustrates a top view of the potential measurement apparatus 500. FIG. 9B illustrates a sectional view taken along the line 9B-9B in FIG. 9A. In the potential measurement apparatus 500, a cradle 504 and two torsion springs 502 and 503 are integrally formed with a bearing substrate. The cradle 504 has two detection electrodes 511 and 512. The cradle 504 is symmetrical about the center line A-A′ in the longitudinal direction of the torsion springs 502 and 503. The detection electrodes 511 and 512 are so arranged as to be line symmetrical with respect to the center line A-A′. In addition, the detection electrodes 511 and 512 are respectively connected, through electrode wirings 513 and 514, to extraction electrodes 515 and 516 provided on the bearing substrate 501. Moreover, the extraction electrodes 515 and 516 are connected to a differential amplifier 520 through wirings 517 and 518.
A potential measurement method with the potential measurement apparatus 500 will be described with reference to FIG. 9B showing a sectional view taken along the line 9B-9B in FIG. 9A. The potential measurement apparatus 500 is set opposite to a measurement object surface 521. The measurement object surface 521 is, for example, a photosensitive drum. In addition, the potential measurement apparatus 500 is placed inside a case 522 which is electrically conductive and electrically grounded. A dashed line in FIG. 9B illustrates a state in which a cradle 504 is shaken by a cradle drive mechanism (not illustrated in the drawing). By periodically shaking the cradle 504, the distance between the detection electrodes 511 and 512 and the measurement object surface 521 and the coupling capacitance therebetween are periodically varied. Thereby, signal current containing information on the potential of the measurement object surface 521 can be taken out of the differential amplifier 520.
In addition, as a unit adjusting the resonance property of an oscillator connected to a fixed part through an elastic supporting part, an optical scanner provided with a spring constant variable element in the elastic supporting part has also been proposed (see Japanese Patent Publication No. 2981600).
The above described potential measurement apparatus involves the following problems. The shaking state of the cradle may change due to changes in environment when being used, which may inhibit the stability of potential measurement being improved. For example, when the temperature of environment when being used changes, the spring constant of the torsion spring may change, and then the resonance frequency may change. In addition, the optical scanner in the afore-mentioned prior art has nothing to do with potential measurement.